1. Field of the Invention
The present invention generally relates to an electro-optical reader for reading indicia and, more particularly, to a point-of-transaction workstation for reading not only one-dimensional indicia, such as bar code symbols, but also for reading two-dimensional indicia, by image capture.
2. Description of the Related Art
Flat bed laser readers, also known as horizontal slot scanners, have been used to electro-optically read one-dimensional bar code symbols, particularly of the Universal Product Code (UPC) type, at a point-of-transaction workstation in supermarkets, warehouse clubs, department stores, and other kinds of retailers for many years. As exemplified by U.S. Pat. Nos. 5,059,779; 5,124,539 and No. 5,200,599, a single, horizontal window is set flush with, and built into, a horizontal countertop of the workstation. Products to be purchased bear an identifying symbol and are typically slid across the horizontal window through which a multitude of scan lines is projected in a generally upwards direction. When at least one of the scan lines sweeps over a symbol associated with a product, the symbol is processed and read.
The multitude of scan lines is generated by a scan pattern generator which includes a laser for emitting a laser beam at a mirrored component mounted on a shaft for rotation by a motor about an axis. A plurality of stationary mirrors is arranged about the axis. As the mirrored component turns, the laser beam is successively reflected onto the stationary mirrors for reflection therefrom through the horizontal window as a scan pattern of the scan lines.
It is also known to provide a point-of-transaction workstation with a generally vertical window that faces an operator at the workstation. The generally vertical window is oriented perpendicularly to the horizontal window, or is slightly rearwardly inclined. The scan pattern generator within the workstation also projects the multitude of scan lines in a generally outward direction through the vertical window toward the operator. The generator for the vertical window can be the same as or different from the generator for the horizontal window. The operator slides the products past either window from right to left, or from left to right, in a “swipe” mode. Alternatively, the operator merely presents the symbol on the product to the center of either window in a “presentation” mode. The choice depends on operator preference or on the layout of the workstation.
Sometimes, the vertical window is not built into the workstation as a permanent installation. Instead, a vertical slot scanner is configured as a portable reader which is placed on the countertop of an existing horizontal slot scanner.
Each product must be oriented by the operator with the symbol facing away from the operator and directly towards either window. Hence, the operator cannot see exactly where the symbol is during scanning. In typical “blind-aiming” usage, it is not uncommon for the operator to repeatedly swipe or present a single symbol several times before the symbol is successfully read, thereby slowing down transaction processing and reducing productivity.
The blind-aiming of the symbol is made more difficult because the position and orientation of the symbol are variable. The symbol may be located low or high, or right to left, on the product, or anywhere in between. The symbol may be oriented in a “picket fence” orientation in which the elongated parallel bars of the one-dimensional UPC symbol are vertical, or in a “ladder” orientation in which the symbol bars are horizontal, or at any orientation angle in between.
In such an environment, it is important that the scan lines located at, and projected from, either window provide a full coverage scan zone which extends down as close as possible to the countertop, and as high as possible above the countertop, and as wide as possible across the width of the countertop. The scan patterns projected into space in front of the windows grow rapidly in order to cover areas on products that are positioned not on the windows, but several inches therefrom. The scan zone must include scan lines oriented to read symbols positioned in any possible way across the entire volume of the scan zone.
As advantageous as these point-of-transaction workstations are in processing transactions involving products associated with one-dimensional symbols each having a row of bars and spaces spaced apart along one direction, the workstations cannot process two-dimensional symbols, such as Code 49 which introduced the concept of vertically stacking a plurality of rows of bar and space patterns in a single symbol. The structure of Code 49 is described in U.S. Pat. No. 4,794,239. Another two-dimensional code structure for increasing the amount of data that can be represented or stored on a given amount of surface area is known as PDF417 and is described in U.S. Pat. No. 5,304,786. Such two-dimensional symbols are generally read by electro-optical readers operative for projecting a laser beam as a raster of scan lines, each line extending in one direction over a respective row, and all the lines being spaced apart along a height of the two-dimensional symbol in a generally perpendicular direction. Also, the turning mirrored component of the scan pattern generators in the known workstations tend to wear out because of its constant motion and need to be replaced from time to time.
The aforementioned Code 49 and PDF417, however, both belong to a special class of two-dimensional symbols, which are especially designed to be able to be scanned by rastering laser scanners. The more general class of two-dimensional symbols, such as DataMatrix, are not designed in a way conducive for laser scanning.
Both one- and two-dimensional symbols can be read by employing solid-state imagers, including those two-dimensional symbols that cannot be read by laser scan pattern generators. For example, an image sensor device may be employed which has a one- or two-dimensional array of cells or photosensors which correspond to image elements or pixels in a field of view of the device. Such an image sensor device may include a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device and associated circuits for producing electronic signals corresponding to a one- or two-dimensional array of pixel information for a field of view.
It is therefore known to use a solid-state device for capturing a monochrome image of a symbol as, for example, disclosed in U.S. Pat. No. 5,703,349. It is also known to use a solid-state device with multiple buried channels for capturing a full color image of a target as, for example, disclosed in U.S. Pat. No. 4,613,895. It is common to provide a two-dimensional CCD with a 640×480 resolution commonly found in VGA monitors, although other resolution sizes are possible.
However, the known point-of-transaction workstations do not read two-dimensional symbols, nor do they utilize solid-state imagers for capturing images of one-dimensional symbols or two-dimensional symbols required to be electro-optically read. To acquire a target image, a solid-state imager, for example as embodied in a consumer digital camera, must be held in a stationary position relative to the target. Only when a solid-state imager is held in a fixed position relative to a target symbol can an image of the symbol be clearly captured and decoded, with the data encoded in the symbol being sent to a host for processing. In the context of a point-of-transaction workstation where the operator swipes the symbol past the window, sometimes once, sometimes several times, and where the operator presents the symbol with an additional component of movement toward and away from a window, and in some cases where the symbols are transported on a moving conveyor past a window, the image of the symbol is blurred due to the relative motion between the symbol and the imager and, as a result, the image cannot be reliably and successfully decoded and read. Also, the field of view of a single solid-state imager is not extensive, thereby compromising the capability of providing a full coverage, light collection zone at the countertop.